Cummins ISC - 700k miles

I thank 700,000 is above average for a 8.3. If you can have an inframe done built into the long term finance you would probably be better off. Take care of the crank, low interest and no worry of that a sudden engine repair. Large sudden expenditure and time off unexpectedly.

 

I spent 5 years with a deleted Isc in a Freightliner M2. Pulled a 4 place car hauler. Always at 52000lbs. Ran great, pulled hard. Then it dropped a valve at 800 000km. Luck of the draw. I've seen almost new engines do that.
If it isn't using oil, or putting coolant in the oil I'd run it nicely till the engine actually needed work.

If it needed replacement, with those kinda miles on it. A drop in would be the way to go. New harness, injectors, turbo all in 1 nice package.

Or find the next decent inexpensive truck to replace it with.